Contrary to the way Hobbes has been interpreted for centuries, I will argue that Hobbes laid the groundwork for contemporary international law and for a distinctly moral approach to the rules of war. The paper has the following structure. First, I will explain the role that the laws of nature play in Hobbes's understanding of the state of war. Second, I will explain Hobbes's views of self-preservation and inflicting cruelty. Third, I reconstruct Hobbes's important insight that rationality governs all human affairs, even those concerning war. Fourth, I explicate the idea of cruelty moving from what Hobbes says to a plausible Hobbesian position. Fifth, I address recent philosophical writing on how best to understand the rules of war. Sixth, I then turn to legal discussions of cruelty's place in debates about the laws of war, showing how my Hobbesian approach can ground these laws.